1. Field of the Invention
The present invention relates to a hydraulic control apparatus of an automatic transmission for controlling an engaging state of a frictional engaging element in the automatic transmission mounted to, for example, a vehicle, by oil pressure, and more particularly relates to a hydraulic control apparatus of the automatic transmission having a switching valve for selectively switching and supplying control pressure controlled by a solenoid valve and line pressure to a hydraulic servo of the frictional engaging element.
2. Description of the Related Art
The automatic transmission mounted to the vehicle conventionally has the hydraulic control apparatus for controlling the engaging state of the frictional engaging element such as a clutch, a brake, etc. by hydraulic control for supplying the engaging state to each of the respective hydraulic servos. When an engaging operation and a release operation of the frictional engaging element are particularly performed in this hydraulic control, it is necessary to finely control the supplied oil pressure by the solenoid valve and a control valve, etc. controlled by the solenoid valve, and prevent a sudden torque change (speed change shock) by smoothly performing the engaging operation and the release operation.
However, for example, when the oil pressure after the engaging operation, i.e., during the engagement, is continuously supplied through the above control valve, there is a fear that running ability is injured if the control valve were to stick. Therefore, a structure for switching the oil pressure (hereinafter set to “control pressure”) controlled by the solenoid valve and the line pressure by the switching valve is proposed (JP-A-2002-266995).
Here, the conventional hydraulic control apparatus will be briefly explained. FIG. 7 is a schematic view showing a hydraulic circuit of the conventional hydraulic control apparatus. FIG. 8 is a time chart showing an experimental example at an N-D shift time using the conventional hydraulic control apparatus. As shown in FIG. 7, modulator pressure PMOD for adjusting line pressure PL adjusted by a primary regulator valve, etc. with respect to the oil pressure from an oil pump operated in association with an unillustrated engine, so as not to further become a predetermined pressure or more, is inputted to a linear solenoid valve 21 for C-1. When the linear solenoid valve 21 for C-1 is operated and controlled, the modulator pressure PMOD is adjusted and signal pressure PSLC1 is outputted from a port 21b. The signal pressure PSLC1 is outputted to an oil chamber 22a of the control valve 22 and an oil chamber 23a of a C-1 apply relay valve 23.
On the other hand, the above line pressure PL is inputted to a port 22c of the above control valve 22 and a port 23d of the above C-1 apply relay valve 23. Further, the control pressure provided by adjusting the line pressure PL inputted to the port 22c of the control valve 22 is outputted from a port 22d and is inputted to a port 23c of the C-1 apply relay valve 23. The above modulator pressure PMOD is inputted to an oil chamber 23b of the C-1 apply relay valve 23.
When the engaging operation of the clutch C-1 is performed, the modulator pressure PMOD is normally inputted to the oil chamber 23b of the C-1 apply relay valve 23. A spool 23p is located in an upper position and ports 23c and 23e are communicated with each other. Thus, the control pressure outputted by the control valve 22 is supplied to a C-1 hydraulic servo 25. However, for example, when the oil temperature is high and an engine rotation number is low and the discharge amount of the oil pump is small, there is a fear that problems are caused.
Namely, as shown in FIG. 8, for example, when a driver sets an unillustrated shift lever to an N-D shift (shifts the shift lever from a neutral range to a drive range) at a time point t1, a so-called back-lash filling operation (fast fill) for making an unillustrated piston of the C-1 hydraulic servo 25 approach until a frictional plate is started at a time point t2. Thus, the linear solenoid valve 21 for C-1 is electronically controlled and operated, and the oil is flowed into the linear solenoid valve 21 for C-1 (and oil paths b, c, d and oil chambers 22a, 23a), and the line pressure PL (modulator pressure PMOD) is reduced during time until a time point t3.
Therefore, the oil pressure of the oil chamber 23b of the C-1 apply relay valve 23 is reduced, and the spool 23p of the C-1 apply relay valve 23 is moved in error to a lower position at the time point t3 by the signal pressure PSLC1 inputted to the oil chamber 23a and the biasing force of a spring 23s. Thereafter, the port 23d and the port 23e are communicated with each other, and the oil is flowed into an operating oil chamber of the C-1 hydraulic servo 25 and the line pressure PL (modulator pressure PMOD) is further reduced (i.e., the spool 23p is located in the lower position as it is). Thus, the piston starting a stroke at a time point t4 is further pressed in addition to the back-lash filling operation, and the clutch C-1 is engaged at a time point t5. Therefore, output torque is changed as shown in FIG. 8. Namely, a speed change shock is caused by engaging the clutch C-1 in the back-lash filling operation in which no clutch C-1 is originally engaged.
It is considered that such a problem is caused at an N-R shift (shift from the neutral range to a reverse range) time, a downshift time of a coast (engine brake) state, a neutral control (control for maintaining the clutch for transmitting power in a state just before the engagement in a running range) time, etc. as well as the N-D shift time. Further, for example, even when the line pressure PL is merely reduced by operating one linear solenoid valve and no switching valve is operated in error, there is also a case in which the line pressure is reduced in flowing the oil into the hydraulic servo of another frictional engagement element and the switching valve is operated in error. A similar problem is also caused in this case.
The above problems can be solved by preventing the reduction in the line pressure PL (modulator pressure PMOD) by increasing the discharge amount of the oil pump, i.e., by preventing the erroneous operation of the switching valve. However, it is necessary to increase an idling rotation number, the size of the oil pump, etc. to increase the discharge amount of the oil pump. Accordingly, these increases obstruct the improvement of fuel cost of the vehicle and compact formation of the automatic transmission.